Keyword URL mapping is the process of determining which page of a website should contain certain keywords to maximize the optimization of the entire website as a whole. A lot of times optimization is only directed at the homepage of a website. This is not the best strategy simply because it is impossible for any one page of a website to be top loaded with several keywords. Each page of a website needs to be optimized for only a few keywords for it to have a chance to rank highly for any one keyword phrase search.
The best practice is to dedicate specific pages of a website to be optimized for specific keyword terms and to thus to spread your keywords out over each page of your entire website. Keyword URL mapping is the process of doing just this.